I. Field of the Invention
The present invention pertains generally to the field of wireless communications, and more specifically to digital combining of forward channels in cellular base stations.
II. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term xe2x80x9ccellularxe2x80x9d systems encompasses both cellular and PCS frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008, etc. (often referred to collectively herein as IS-95), are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. An exemplary cellular telephone system configured substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459, which is assigned to the assignee of the present invention and fully incorporated herein by reference. The aforesaid patent illustrates transmit, or forward-link, signal processing in a CDMA base station. Exemplary receive, or reverse-link, signal processing in a CDMA base station is described in U.S. application Ser. No. 08/987,172, filed Dec. 9, 1997, entitled MULTICHANNEL DEMODULATOR, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same RF band. For example, each mobile subscriber unit (typically a cellular telephone) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with mobile units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum. It is to be understood that while 1.25 MHz is a preferred CDMA channel bandwidth, the CDMA channel bandwidth need not be restricted to 1.25 MHz, and could instead be any number, such as, e.g., 5 MHz.
Transmitting signals over the same RF spectrum provides various benefits including, e.g., an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. (In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station.) An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In conventional cellular telephone systems, a public switched telephone network (PSTN) (typically a telephone company) and a mobile switching center (MSC) communicate with one or more base station controllers (BSCs) over standardized E1 and/or T1 telephone lines (hereinafter referred to as E1/T1 lines). The BSCs communicate with base station transceiver subsystems (BTSs) (also referred to as either base stations or cell sites), and with each other, over a backhaul comprising E1/T1 lines. The BTSs communicate with mobile units (i.e., cellular telephones) via RF signals sent over the air.
In conventional systems, base stations, or cell sites, are configured to communicate via an over-the-air interface with various mobile units. In CDMA cellular systems, the base stations (sometimes referred to herein as base station transceiver subsystems (BTSs)) are often segmented into sectors, as defined by directional antennas, to increase the capacity of the cell. The sectors themselves may be referred to as cell sites. Conventional base station architectures typically employ three such sectors, with the radial directions each sector antenna points differing by 120 degrees. Each sector in a CDMA system functions, for network purposes, as an independent base station. It would be desirable, therefore, in the interest of improving system capacity, to increase the number of sectors in a base station architecture without sacrificing reliability or efficiency and without adding to the manufacturing cost of the base station. It would further be advantageous to flexibly increase the number of sectors in the base station, and to increase the number of frequency assignments.
Increasing the capacity of a cell can be accomplished by using additional 1.25 MHz bands of spectrum. This approach has the benefit of not requiring additional antennas if front-end combiners are used to combine frequency outputs of single-carrier amplifiers, or if multicarrier amplifiers are used. In practice, however, both increasing the number of sectors and increasing the number of frequency bands are necessary to support large call-carrying capacities at a single cell site.
Conventional base stations are relatively large, heavy, and expensive to build. The base stations represent the primary infrastructure of a cellular system, and as such they contribute significantly to the cost to implement such a system, and to the reliability and maintainability of such a system. Further, while the placement of base stations in such systems must comport with network planning, it is desirable that the base stations be physically situated to be as unobtrusive as possible. Hence, there is an ongoing drive in the industry to reduce the size and cost of base stations without sacrificing their reliability and maintainability. It would therefore be advantageous to design a base station architecture of significantly reduced size and cost. It would further be advantageous to provide a base station architecture that improves the reliability and maintainability of the base station.
Many conventional base station architectures are centered around integrated modulator/demodulators known as cell site modems (CSMs). While CSMs can handle multiple channel elements, each channel element can process only one call at a time. Thus, in reverse-link (mobile-to-BTS) signal processing at a conventional base station, an antenna system receives a set of reverse-link signals transmitted in the same RF band from a group of mobile units in the associated coverage area. An RF receiver downconverts and digitizes the set of reverse-link signals, yielding digital samples that are received by the CSMs. Each CSM (or, in the case of a multi-channel-element CSM, each channel element of a CSM) is allocated by a controller to process a particular reverse-link signal from a particular mobile unit, and each CSM generates digital data that is forwarded to the base station controller. An ideal base station architecture must be capable of supporting up to a maximum of sixty-four mobile units per sector. This prevents the network from being hardware-limited when the air link may be able to support up to sixty-four users per sector. (Consider, e.g., a wireless local loop (WLL) environment, in which stationary subscriber units provide a hardware constraint.) Hence, a base station could contain up to sixty-four channel elements per sector. Such base stations have been implemented and deployed on a wide scale, but at a relatively high cost. One of the main contributors to this cost is the complexity and sensitivity of the necessary interconnects between the RF unit and the various CSMs, and between the base station controllers and the CSMs. By way of example, in conventional base stations a subset of twenty-four or more channel elements is typically placed on a circuit board, and a set of circuit boards is coupled via a backplane, which in turn is coupled to an RF unit with sets of coaxial cables. Such interconnecting is expensive due to the amount of cabling necessary and the need to cool the CSMs, and contributes substantially to the overall cost, complexity, and maintenance of the base station. It also places a limit on the number of sectors of a base station with which a mobile unit may be in softer handoff.
In the foregoing respects it would be advantageous to provide a fundamentally improved base station architecture. To this end it would be desirable to provide a system of modulation and demodulation that provides a more efficient base station architecture, flexibly supports multiple frequency assignments and an increased number of sectors, reduces the size and manufacturing cost of the base station, and permits the orderly exchange of power-control information between separate transmit and receive systems of the base station. An exemplary base station embodying such a system is described in a related U.S. application Ser. No. 09/172,719, filed Oct. 13, 1998, entitled BASE STATION ARCHITECTURE and assigned to the assignee of the present invention.
To enhance the capacity of the base station, it is advantageous to transmit more than one forward channel on the same transmit antenna of a sector. This is accomplished conventionally in one of two ways. A multicarrier (i.e., multi-forward-channel) amplifier may be used. Alternatively, two or more single carrier (i.e., single forward-channel) amplifiers may be used, with the outputs of the amplifiers being combined by a combining network such as, e.g., a multiplexer. The approach of combining the outputs of two or more amplifiers is less complicated than using a single, multicarrier amplifier because single carrier amplifiers are easier to design and implement than multicarrier amplifiers. However, the combining network causes an inevitable, and undesirable, loss. Hence, the multicarrier-amplifier approach, while more complex, typically offers greater performance (i.e., more output power). It would be desirable, therefore, to provide a method of using a multicarrier amplifier at lower cost. Thus, there is a need for a method that simplifies the generation of the input waveform to an amplifier.
The present invention is directed to a method that simplifies the generation of the input waveform to an amplifier. Accordingly, in one aspect of the invention, a method of combining forward channels in a base station advantageously includes the steps of digitally upconverting by a different amount each of the output signals generated by two or more modulators in the base station, and summing the resultant upconverted signals to produce a summed signal. In another aspect of the invention, a modulator advantageously includes a first modulation section operational to generate a first output signal, and a second modulation section coupled to the first modulation section and operational to generate a second output signal, digitally upconvert the second output signal, and sum the digitally upconverted second output signal and the first output signal.